242 Mr. J. Young on the Sfriicture of 



instance being exactly that of the thickness of the intervening 

 deposit. In tangential sections of the organism taken 

 below its outer surface we also find that where the sections 

 happen to pass horizontally through any of the layers of 

 sclerenchynia there the mesopores disappear, their walls 

 not extending either upwards or downwards through this 

 deposit ; but as the obscuring layer passes out of section, as 

 happens in most instances, owing to the curvature of the 

 layers around the specimen, the mesopores are always dis- 

 tinctly visible over those parts where the sclerenchyma is 

 absent, or, owing to the darker colour of their cell-walls, are 

 to be seen shining through a thin layer of the sclerenchyma 

 at its junction with the mesopores. 



In connexion with this sclerenchyma-deposit I have also to 

 note the fact that its whole structure is pervaded by a system 

 of very minute pores or foramina which are distinctly visible 

 in all the better-preserved s])ecimens of the organism. This 

 character in F. incrustans I first observed on the surface of 

 some specimens from the limestone-shales of the Lanarkshire 

 coal-field, and noticed in a paper in the Glasg. Geol, Soc. 

 Trans, vol. vii. p. 2-J6 (1883). I have also, since then, proved 

 their existence in all the inner layers of sclerenchyma by 

 ]uimerous sections that I have prepared. 



"When seen on the sui face-layer these pores appear as very 

 minute slightly raised tubercles, whilst in the transparent 

 sections they are seen as tubes or foramina, according as 

 they are viewed in vertical or transverse positions. 1 am 

 glad to find that J\lr. Ulrich, in the paper to which I have 

 already referred, both mentions and figures this perforated 

 structure as being found in one or two forms of American 

 Bryozoa, one of which is the Fistuliporal clausa, Ulrich. In 

 writing on this species he says (p. 47), " My tangential sec- 

 tions do not show ijositivelij that this deposit (sclerenchyma) 

 was perforated ; but judging from the evidence at hand, and 

 especially that afforded by one or two vertical sections, 

 and the very minute pits shown in many specimens, I should 

 say that sue h was actually the case, and I do not doubt that 

 I will yet find a section that will show it in an unquestionable 

 manner." I may here state that I have found clear evidence 

 of this perforated structure in the sclerenchyma-layers of all 

 those specimens in which the calcite has not been too much 

 altered through crystallization. When such is the case the 

 foramina are generally obliterated or are, as in some sections, 

 only faintly visible in parts, where it requires the practised 

 eye to detect them under the microscope. 



In the paper by Messrs. Nicholson and Foord they notice 



